For a while S&W made a SXS that was steel shot compatible. I think it was called the elite gold model and CDNN clearance them out for a little over $1000. Might want to contact them and see if they still have any.
Personally, if I was just going to occasionally hunt waterfowl with a SXS I wouldn't worry about the steel shot compatible and just pick up some bismuth shot (or equivalent) for the few times I took it out. It would give you a lot more choices.
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